A. Field of the Invention
The present invention relates to glasses suitable for being shaped into small, flat windows used as substrates onto which thin film coatings of a suitable material are applied to form optical interference filters. More particularly, the invention relates to a glass composition in which relative proportions of components thereof may be varied to adjust the thermal expansion coefficient of the glass to relatively high values which are tailored to suit characteristics of particular thin film coatings, enabling fabrication of optical interference filters having transmission bands which shift minimally in wavelength in response to varying temperatures and other environmental stresses.
B. Description of Background Art
Optical interference filters fabricated by applying onto a thin, flat transparent glass substrate one or more coatings of dielectric or conductive films each having a thickness which is a multiple of one-quarter wavelength of a light of a particular wavelength are well known and widely used. Optical thin film interference filters are of various types, including low pass or high pass filters which transmit light having wavelengths longer or shorter than a particular cut-off wavelength, band pass filters which transmit only light having wavelengths within a particular band of wavelengths, and notch filters which transmit light over a range of wavelengths comprising a pass band while reflecting or absorbing light in a smaller range of wavelengths centered about a notch wavelength contained within the pass band. One common example of a thin film optical interference filter is the optical anti-reflection coating on the lenses of binoculars, which forms therewith a relatively wide band pass filter for light within the visible spectrum, minimizing reflections from the surface of objective lenses of light received from an object viewed and maximizing transmission of light through the eyepieces to the eyes of a viewer.
Performance requirements for band pass filters used in certain optical communications applications, such as Dense Wavelength Division Multiplexing (DWDM) are more demanding than those imposed on other applications for optical interference filters, as will now be explained.
In Dense Wavelength Division Multiplexing (DWDM) light energy in discrete wavelength bands is modulated with radio frequency signals containing audio, video or digital information including telephone conversations, television transmissions and digital computer data signals. In a typical DWDM system, an optical signal generated by a laser and having a center wavelength in the near infrared portion of the electromagnetic spectrum, e.g., 1.5 microns, (1500 nanometers) is subdivided into a plurality of wavelength bands which comprise separate optical carrier channels. Each optical carrier channel may have a bandwidth of about 0.2 nanometers, and be separated from one another by about 8 nanometers. The amplitude or phase of each of the optical carrier channel signals is modulated by a plurality of radio frequency sub-carrier channel signals, e.g., having a bandwidth of about 25 GHz and a channel separation of about 100 GHz. Each RF channel is in turn modulated at a lower frequency with information such as digitized telephone conversation signals, television signals or other digital data.
In a simplified example, a DWDM system may employ separate optical carrier channel signals each having a band width of 0.2 nanometer and center wavelengths of 1500, 1499.2, 1498.4, 1500.8, and 1501.6, nanometers, respectively. The plurality of optical carrier channel signals is optically combined or xe2x80x9cmultiplexedxe2x80x9d onto a single optical beam, which may then be transmitted on a single optical fiber. Optical multiplexing may be performed using a resonant cavity filter, such as the one depicted in FIG. 3 of U.S. Pat. No. 5,953,134, the entire specification of which is incorporated herein by reference. The resonant cavity filter described therein employs a plurality of individual interference filters, each one being highly transmissive to light in a particular wavelength band, and highly reflective to all other wavelengths of light.
At the receiving end of an optical fiber or other transmission media through which combined or multiplexed optical signals are transmitted, a resonant optical cavity provided with a separate interference filter for each optical channel may be used to divide or xe2x80x9cde-multiplexxe2x80x9d the combined signal into separate optical beams which are arranged to impinge on a plurality of separate photo-detectors, one for each optical channel, thus allowing signal information contained on each optical carrier channel to be directed to appropriate destinations for the information signals on each channel, where the information may be recovered by demodulating the optical carrier signal.
Because of the very narrow bandwidth and close center wavelength spacing required of interference filters used for DWDM applications as described above, both the center wavelength and bandwidth must remain precisely fixed in spite of variations in ambient temperature, humidity, and other environmental conditions encountered by DWDM systems. Otherwise, data transmitted over adjacent optical channels could intermix, be reduced substantially in signal-to-noise ratio, or be lost entirely. Thus, the glass which is used for substrates onto which dielectric coatings are applied to form interference filters for use in DWDM applications must have properties which differ from those of existing glass compositions, for the following reasons.
Conventional glasses may be broadly categorized as xe2x80x9csoftxe2x80x9d or xe2x80x9chard.xe2x80x9d Soft glasses typically have a linear coefficient of thermal expansion (xcex1 or xe2x80x9cCTExe2x80x9d) of greater than 60xc3x9710xe2x88x927, while hard glasses usually have a CTE of less than 60xc3x9710xe2x88x927. The softer glasses generally have a lower Young""s modulus and are generally more subject to surface degradations by environmental conditions such as high temperatures, humidity and/or corrosive atmospheres. On the other hand, it has been determined that glass used as a substrate for receiving dielectric coatings to form highly wavelength-stable interference filters of the type required for DWDM applications should have a relatively high Young""s modulus, to provide required dimensional stability, but must also have a thermal coefficient of expansion which is substantially larger than that typical of hard glasses. Moreover, it has been found that to maintain the center wavelength and bandwidth of optical interference filters stable enough for use in DWDM applications, the thermal expansion coefficient of the glass filter substrate must be rather precisely tailored to suit properties of the particular dielectric coatings applied to the substrate. Typical dielectric coating materials include oxides of titanium, tantalum, niobium, silicon and aluminum, and other substances. It is believed that better wavelength stability is obtained using glass substrates with higher coefficients of expansion, because the high CTE""s tend to produce compressive stresses in metal oxide coatings deposited on the glass, when the glass cools down to ambient temperature after being heated to a temperature typically exceeding 200xc2x0 C. during the coating process, which is typically done in a low pressure chamber.
In apparent recognition of the desirability of providing a glass with special properties for use as substrates for DWDM interference filters, NA0YURI, in Patent Publication Number EP 1081512 disclosed a glass for a light filter stated to be capable of preventing variations of refractive index in a band pass filter, to have a coefficient of thermal expansion within a range from 90xc3x9710xe2x88x927/xc2x0 C. to 120xc3x9710xe2x88x927/xc2x0 C. within a temperature range of xe2x88x9220xc2x0 C. to +70xc2x0 C., and, preferably a Young""s modulus of 75GPa or over, a Vickers hardness of 550 or over, and light transmittance for plate thickness of 10 mm of 90% or over within a wavelength range of 950 nm to 1600 nm.
The present invention was conceived of to provide an improved glass composition which is particularly well suited for use as a substrate for optical interference filters, of the type used in dense wave division multiplexing (DWDM)
An object of the present invention is to provide a glass composition which has a relatively high thermal coefficient of expansion.
Another object of the invention is to provide a glass composition which has a relatively high thermal coefficient of expansion, and a relatively high Young""s modulus.
Another object of the invention is to provide a glass composition which has a relatively high coefficient of thermal expansion, a relatively high Young""s modulus, and a relatively high light transmittance.
Another object of the invention is to provide a glass composition which is suitable for use as a substrate to receive thin film coatings which form in combination with the substrate an interference filter having a transmission band which is stably positioned at a selected wavelength region.
Another object of the invention is to provide a glass composition which has a relatively high thermal coefficient of expansion which may be varied in a predetermined manner as a function of variations in percentage composition of components of the glass.
Another object of the invention is to provide a glass composition which is suitable for use as a substrate for optical interference filters having a transmission band which is stably centered on a particular wavelength.
Another object of the invention is to provide a glass composition which is suitable for use as a substrate for optical interference filters of the type used in Dense Wave Division Multiplexing (DWDM), the composition having a thermal coefficient of expansion adjustable by adjusting proportions of components of the composition to values in the approximate range of 105 to 120xc3x9710xe2x88x927/xc2x0 C. over a temperature range of approximately xe2x88x9230xc2x0 C. to +70xc2x0 C., a Young""s modulus of 85 Gpa or greater, an optical transmittance of 91% or greater over a wavelength range from about 1300 nm to about 1600 nm, for a substrate having a thickness of 8 mm or less, and environmental resistance to surface damage when exposed to high temperature, high humidity environments for extended periods of time.
Various other objects and advantages of the present invention, and its most novel features, will become apparent to those skilled in the art by perusing the accompanying specification and claims.
It is to be understood that although the invention disclosed herein is fully capable of achieving the objects and providing the advantages described, the characteristics of the invention described herein are merely illustrative of the preferred embodiments. Accordingly, I do not intend that the scope of my exclusive rights and privileges in the invention be limited to details of the embodiments described. I do intend that equivalents, adaptations and modifications of the invention reasonably inferable from the description contained herein be included within the scope of the invention as defined by the appended claims.
Briefly stated, the present invention comprehends a new silicate-based optical glass composition which has physical properties that make the glass particularly well suited to use as a substrate on which thin film coatings may be applied to form optical interference filters having a narrow wavelength transmission band which is precisely centered on a selected wavelength, in spite of variations of ambient temperature. Stable narrow band optical interference transmission filters of this type are required for certain optical communication applications such as Dense Wave Division Multiplexing (DWDM). According to one aspect of the present invention, a silicate glass composition is provided which has a relatively large coefficient of thermal expansion, found necessary to maintain wavelength stability in optical interference filters fabricated by applying thin dielectric coatings onto a glass substrate . According to the invention, relative proportions of components of the novel glass composition may be varied to vary the resultant coefficient of thermal expansion (CTE) over an approximate range of 91 to 120xc3x9710xe2x88x927/xc2x0 C. and over a preferred range of approximately 105 to 120 10xe2x88x927/xc2x0 C. According to another aspect of the invention, a silicate glass composition for optical interference filter substrates is provided which has a high CTE, in the approximate range of 91 to 121 10xe2x88x927/xc2x0 C., and also has a high rigidity, i.e., has a Young""s modulus of over 85 Gpa. A high Young""s modulus is desirable for glass substrates used for optical interference filters of the type used in DWDM, since such filters are made using very small, thin plates having dimensions of the order of 2 mmxc3x972 mmxc3x972mm or smaller, and are therefore subject to deformation sufficient to cause shifts in the optical properties of the filter in response to small mechanical stresses, if the Young""s modulus of the glass is not sufficiently high.
According to another aspect of the present invention, a novel silicate glass composition is provided which has a high CTE which may be varied in a predictable manner by changing relative proportions of components of the composition, a high Young""s modulus and a relatively high optical transmittance, e.g., a transmittance of 90% or greater for a sample thickness of 8 mm over the approximate near infrared (NIR) wavelength range of 1300 nm to 1600 nm, while also being resistance to surface damage when exposed to high temperatures and high humidity conditions for prolonged time periods.
The high thermal expansion coefficient range stated above as being a requirement for glass used as a substrate for DWDM optical interference filters is higher than that of most current optical glasses. Typically, such higher thermal coefficients of expansion are achieved only in softer, weaker glasses. But softer, weaker glasses have a lower Young""s modulus, and are therefore insufficiently rigid for DWDM applications, and also are subject to surface degradation in high humidity, high temperature environments. Accordingly, a delicate manipulation of glass composition is required to create a glass which has the properties specified above.
It has been discovered by the present inventor that requirements of glass properties for DWDM filter substrates set forth above can be fulfilled with glasses having the following composition (in mole %):
40-62% SiO2 
2-20% (Al2O3, B2O3, La2O3)
8-36% alkaline oxides
0-40% alkaline earth oxides
0-20% of any non-color generating heavier metal oxides.
A preferred glass composition found by the present inventor to be capable of satisfying the aforementioned requirements has the following composition, in mole %:
43.3% SiO2 
7.0% Al2O3 
12.7% CaO
7.3% SrO
7.8% Li2O
13.2% Na2O
8.0% K2O
0.7% ZrO2 
0.1% Sb2O3.
The preferred composition glass had a coefficient of thermal expansion (CTE) of 112xc3x9710xe2x88x927/C over the temperature range xe2x88x9230xc2x0 C. to +70xc2x0 C., a Young""s modulus E of 88.3 Gpa, and an optical transmittance of 90.9% at 1550 nm for an 8 mm thick sample.